In U.S. Pat. No. 4,711,746 issued Dec. 8.sup.th, 1987, the present inventor disclosed a device for forming a thermoplastic weld between two plastic parts in which a thermoplastic material is forced into a chamber and melted within the chamber so as to generate a molten plastic material under a pressure of at least 1000 psi and more preferably of the order of 2500 psi. The molten plastic is expelled through an orifice so that the molten plastic under pressure from the heating chamber is forced into the space between two plastic components to effect a weld therebetween. It will be appreciated that the pressure set forth above is orders of magnitude higher than the pressure necessary for a conventional simple glue gun in which a hot melt adhesive is expelled simply under low or zero pressure.
The thermoplastic welding system disclosed in the above patent has achieved considerable commercial success. However in order to successfully operate the device it is necessary to inject into the heating chamber a thin rod or wire of the thermoplastic material so that the pressure generated within the chamber is obtained by forcing the rod into the chamber with a longitudinal force which is sufficient to generate the required pressure. The thermoplastic material in rod form generally has a diameter of the order of one sixteenth to one quarter inch and more preferably of the order of one eighth inch. This relatively small diameter reduces the longitudinal force necessary on the rod to generate the required pressure but makes the rod more pliable and difficult to feed.
The original patent disclosed a technique for feeding the rod in which the rod passes between two feeding disks with the rod tangential to each of the disks and lying in a common plane with the disks. Each disk has a groove in its outer periphery so as to attempt to trap the cylindrical rod between the rollers. However in practice this arrangement was ineffective and was replaced by a reciprocating clamping arrangement which acted to feed or stuff the pliable rod into the tube leading to the chamber. This device using the reciprocating clamp arrangement has achieved commercial success, but the rod feed has remained a source of concern and potential problem in that it is relatively expensive, complex and prone to breakdown.
Attempts have been made to replace the reciprocating clamp of the commercial embodiment and one attempt is set forth in U.S. Pat. No. 5,971,212 of the present inventor which is issued Oct. 26.sup.th, 1999. This device provides a friction block on one side of the rod which reciprocates back and forth so as to feed the rod forwardly along a channel. The use of the reciprocating drive arrangement however requires a component to prevent reverse movement of the rod when the reciprocating device reverses to commence a further feeding stroke. This device unfortunately has failed to overcome the above problems and has not been adopted.